The present invention relates to a method for damping relative movements occurring between a first and a second part of a work vehicle during advance of the vehicle, which parts are interconnected by means of at least one hydraulic actuator.
The term work vehicle comprises different types of material handling vehicles like construction machines, such as a wheel loader, a backhoe loader, a motor grader and an excavator. The invention will be described below in a case in which it is applied in a wheel loader. This is to be regarded only as an example of a preferred application.
Said actuator may be a linear actuator in the form of a hydraulic cylinder. A wheel loader comprises several such hydraulic cylinders in order to perform certain work functions. A first pair of hydraulic cylinders is arranged for turning (steering) the wheel loader. A second pair of hydraulic cylinders is arranged for lifting load-arm unit and a further hydraulic cylinder is arranged on the load arm unit for tilting an implement, for example a bucket or forks, arranged on the load arm unit.
It is known to damp relative movements between a load-arm unit and a forward vehicle section in a wheel loader by means of a load-arm unit suspension system.
By damping the movements, the comfort of the driver in the machine is increased and material that is being carried by the implement is prevented from leaving the implement. If, for example, a bucket is arranged on the load-arm unit, it is desirable that the material that is loaded in the bucket does not fall out of the bucket when the vehicle goes over a bump. A vehicle provided with large tyres uses the tyres as springs during advancing over an uneven surface. However, the tyres are not capable of effectively damping the jumping movements and pitching oscillations that occur in the vehicle body when the vehicle travels on an uneven surface.
When the vehicle goes over a bump in the surface, the vehicle body moves upwards. On account of the mass inertia in the load-arm unit, the load-arm unit tends to stay at its existing level above the surface. Instead of the load-arm unit following the vehicle body upwards, the pistons of the cylinders are forced into the cylinders, which means that hydraulic oil flows out of the cylinders.
According to a known suspension system, see for example WO 99/16981, an accumulator is arranged downstream of the cylinders. The gas present in the accumulator will thus be compressed when the vehicle goes over a bump. The pistons will be displaced into the cylinders as long as the pressure in the cylinders is lower than the pressure that is needed in order to overcome the accelerating force and the force of gravity from the load assembly. When the machine goes over a hole in the surface, the reverse sequence occurs, that is to say that hydraulic oil flows from the accumulator to the cylinders.
During work with the machine in, for example, a gravel pit, the load arm suspension system should be deactivated when the bucket is to be filled. The vehicle then drives with great force into a gravel heap, with the bucket located in front of it. It is then desirable that the load-arm unit is rotationally rigid and that the pistons in the cylinders maintain their set position. Subsequently, when the machine is to transport the gravel in the bucket, the load-arm unit suspension system is activated. On activation of the load arm suspension system, the load arm assembly is to maintain its set position.
The known suspension system in WO 99/16981 is passive. The lift cylinders are connected to at least one gas filled accumulator, which functions as a spring. The hydraulic fluid flowing with a certain pressure drop back and forth between the cylinder and the accumulator functions as a dampener. Further, friction in the load-arm unit and the cylinder contribute to the dampening function.
Further, when traveling over an uneven ground, large side acceleration forces are created around a steering joint in a frame steered vehicle, like a wheel loader. It is known to use accumulators in fluid connection to the steering cylinders, which accumulators have the effect of damping the relative movements between the frame parts also when the steering cylinders are not activated for steering the vehicle.
It is desirable to achieve a method for dampening the relative movements between a first and a second part of the work vehicle during advance of the vehicle, which recovers energy from said relative movements.
The method according to an aspect of the present invention comprises the steps of detecting at least one operation parameter of the vehicle, determining whether a damping condition is present based on the detected operation parameter and if the determined damping condition is present: controlling a displacement of a variable displacement hydraulic motor arranged downstream of the actuator in such a way that energy from a hydraulic fluid transmitted from the actuator is converted to a rotational energy in the motor, and transmitting the recovered energy from the motor to a power source.
Thus, the kinetic energy from undesired relative movements of the vehicle, like rocking or oscillating movements, will be recovered by this method. Further, by virtue of this energy recovery method, fuel consumption is reduced.
The method may be used for damping and energy recovery from several functions of the vehicle, like from the lifting function, tilting function and steering function. Relative motions may be damped and energy may be recovered simultaneously with a suitably designed system.
According to an aspect of the invention, the detected operation parameter is an angle between the first and second parts and/or a hydraulic fluid pressure associated to the actuator.
For example, the damping condition may be determined based on a predetermined change in said detected hydraulic fluid pressure. Further, one may determine if the desired pressure change is present for initiating an energy recovery phase by determining the frequency content of the detected pressure and/or a derivative of the detected pressure. In this way, it is determined if there is an operation condition present, such as a permanent rise in the pressure due to a heavier load or a temporary disturbance, which will not initiate an energy recovery phase.
According to an aspect of the invention, the method comprises the step of automatically moving the first and second parts relative to one another to a specific relative position, preferably by means of said actuator, after termination of an energy recovery phase. Said specific relative position is preferably the initial relative position of the two parts prior to initiation of the energy recovery operation.